Typically transmissive displays are illuminated with a backlighting system where light from the backlighting source passes through the transmissive display towards a viewer. Reflective displays cannot use a conventional backlighting system due to blockage of the backlight by the non-transmissive (i.e., reflective) display. Therefore, front-lighting systems have been developed for reflective displays. These front-lighting systems include a light source at the edge of the reflective display and a light-guiding system with a light guide on top of the display, for example. The light guide has out-coupling structures on its surface for coupling the light out towards the reflective display. This way, the light is evenly spread over the complete display area and reflected towards the viewer.
Performance of front-lighting systems for reflective displays is poor compared to back-lighting systems for transmissive displays due to contrast degradation by unwanted out-coupling of light directly towards the viewer, instead of being reflected first from the reflective display, for example.
For reflective liquid crystal displays (LCDs), which are currently the dominant display technology, a good alternative for the currently used front-lighting systems is not available. One reason is because the light has to be coupled into the reflective LCD at an almost perpendicular angle to the display surface. Otherwise, grey level shifts occur due to viewing angle problems that are inherent to LCD effects.
For electrophoretic displays, the situation is different. The display effects of electrophoretic displays do not have the viewing angle problems associated with LCD displays. Rather, the electrophoretic displays are almost perfect (Lambertian) diffuse scatterers of light. Therefore, the front-light does not have to be coupled into the electrophoretic display at a perpendicular angle to the display surface. This opens new ways of making a good front-lighting system for electrophoretic displays that may be flexible displays. Conventional electrophoretic cells are described in U.S. Pat. No. 6,906,851 to Yuasa, and U.S. Patent Application Publication No. 2005/0179852 to Kawai, each of which is incorporated herein by reference in its entirety.
Flexible displays including rollable or wrappable displays are expected to become more commonplace. Wrappable or rollable displays may be manufactured using a variety of display elements, e.g. LCD elements or Organic Light. Emitting Display (OLED) elements, deposited on or sandwiched between flexible substrates, e.g. polymer substrates. Light emitting polymers offer various advantages such as excellent viewing angle and contrast, as well as low power consumption.